The upright boom of a large tower crane can extend to a height of well over 150 ft. (45 m.), and the boom that projects forward from the tower may have a length which is only slightly less than the hei9ht of the tower. When the crane is working, the boom is swung up and down through an operating range that can carry it from about 15.degree. to about 75.degree. above the horizontal, for moving a hoisted load toward and from the base of the crane. Because of the height of the tower and the length of the boom, it is desirable to enable the boom to be swung to an inoperative parking position in which the boom extends down alongside the tower. In that position of the boom, high winds exert less force and leverage against it, so that the crane is less likely to be tipped over by a high wind. The boom is also brought to this inoperative position when the tower is to be lowered, the tower then being swung forwardly down to a horizontal position in which the boom underlies it.
For the boom to be brought to its inoperative position, it must swing down from its lowermost working position about an axis that is at least a short distance in front of the front side of the tower. In its range of working positions, however, the boom should preferably swing about an axis that is centered on the top of the tower--that is, midway between the front and rear sides of the tower--so that the forces that the boom exerts upon its pivot axis do not impose bending moments upon the tower.
In most prior tower cranes, the axis about which the boom swung through its range of working positions was also the axis about which it swung to and from its inoperative position, and that axis was located at or slightly in front of the front side of the tower. To support the bending loads that are imposed upon the tower with the boom axis in this off-center location, the tower must be built with substantially greater strength than would be needed if the boom pivot axis were centered over the tower in the working range of the boom, thus increasing the cost of the tower and raising the center of gravity of the crane as a whole.
Some difficult problems are involved in shifting the axis about which the boom swings, because all control of the boom must be exerted from near the bottom of the tower. For safety, it should not be possible to lower the boom below the lower part of its working range unless its swinging axis has been shifted to the forward position, and the boom should not be allowed to swing in its working range until its swinging axis has been shifted back to where it is centered over the tower. Obviously, any expedient for shifting the boom axis must be so sturdy and simple as to be extremely reliable, but should nevertheless be inexpensive and relatively light in weight.